There is an increasingly strong trend to replace glass, metal, and paper containers with plastic containers to package goods for storage or sale, e.g., milk and juice containers. The driving force behind this trend includes many factors, e.g., improved energy efficiency in producing the containers, customer preference, reduced product loss due to breakage, reduced shipping costs, improved storage geometry, and improved storage performance. In general, the plastic containers are prepared by known techniques from a multilayer laminated film structure comprising (a) a barrier layer(s), (b) a structural component, (c) a sealant layer, (d) an adhesive layer and, optionally, (e) a scrap layer.
Most such structures contain one or more barrier layers designed to keep the contents of the package or container effectively within the confines of the package or container and to keep the external environment, such as oxygen or water, from entering the contents. The purpose of the structural component is usually to provide some means of supporting the product being contained. The sealant layer provides a means of closing (i.e., sealing) the package after the contents of the package or container has been introduced. Generally heat and pressure are used to close or seal the package or container although more recent technological advances use radio frequency or ultrasonic sealing means and techniques. A scrap layer (e.g., reground, off-grade polymer) can be and typically is used in the multilayer structure. The scrap layer may act as a structural component, but the main function of the scrap layer is to improve the economics of producing the package. The adhesive component holds all of the other components together, giving the package or container structural integrity. The external layer can be printed for product identification, and may be any of the layers discussed above. Typical multilayer film manufacturing techniques utilize cast and/or blown multilayer film.
Many adhesive layers are resin blends of thermoplastic polymers. The thermoplastic polymers form a known class of compounds which includes, for example, high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and ethylene/vinyl acetate (EVA) copolymers. The thermoplastic polymers can also have various unsaturated carboxylic acids, carboxylic acid anhydrides, or other acid derivatives grafted onto, or polymerized into the polymer. The purpose of the grafted portion of the resin is usually to enhance the adherence of the polymer to a substrate, especially polar substrates such as polyesters, polyamides, or ethylene/vinyl alcohol (EVOH) copolymers.
Mito et al. in U.S. Pat. No. 4,370,388 (Mitsui) describes a laminated multilayer structure which contains as one of the layers an adhesive composition consisting of (a) HDPE grafted with a dicarboxylic acid or functional derivatives there-of, (b) ethylene/4-methyl pentene copolymer, and (c) a "rubbery" synthetic polymer, with ethylene/propylene copolymers preferred as the rubbery polymer.
Another example, Adur et al. in U.S. Pat. No. 4,537,836 (Chemplex) describes a composite structure wherein a three component resin blend consisting essentially of (a) a graft copolymer, (b) LDPE, LLDPE, or mixtures thereof, and (c) a homopolymer or copolymer containing greater than 50 weight percent of an alpha olefin of 4-15 carbon atoms was alleged to be useful as the adhesive layer.
Yet another example of a laminated multilayer structure is disclosed in Nagano, U.S. Pat. No. 4,397,916 (Mitsui) wherein one of the layers of the multilayer structure was an adhesive layer comprising (a) an ethylene/alpha olefin copolymer blended with (b) a graft-modified ethylene/alpha-olefin copolymer having a carboxylic acid content between 0.01 to 10% by weight. The alpha-olefin was said to have from 3-30 carbon atoms.
While many of these polymer blends are taught to be useful as adhesive components in multilayer structures, there continues to be a commercial need for an adhesive with superior properties and/or better economics.